Custom Sky Textures

Note that this feature has been removed with Pro PE 3.0

A mission designer may pick a custom sky texture, fitting the scenario in question. Use the "Options -> Sky Texture.." menu item in the Mission Designer (or the Alt-S hotkey while in the 3D view) to pick a texture.

It is also possible to create completely new sky textures, the following things are important:

• The center of the texture orients south; west is at the left quarter, east is to the right of the center.
• The lowest line determines the color of the fogging that is applied to the scene, so it should be a nearly neutral grey, usually.
• Texture dimensions should be 2:1, e.g. 1024x512.

(source)

It seems that these textures need to the placed into the "eSim Games\SB Pro PE\textures\sky" subdirectory of your PRO PE installation folder.